A typical piston type internal combustion engine (reciprocating engine) is described first. FIGS. 4A to 4C show a four-cycle internal combustion engine, which typically includes a cylinder 1 (defining a cylinder bore), a piston 2 slidably mounted in the cylinder bore of the cylinder 1, and a connecting rod 4 through which the piston 2 is connected to a crankshaft 3. This engine further includes intake and exhaust valves 5, and a spark plug 6.
This internal combustion engine is configured such that when the piston 2 moves down from the uppermost position (FIG. 4A) to an intermediate position (FIG. 4B) and then to the lowermost position (FIG. 4C), a fuel-air mixture is introduced (sucked) into the cylinder 1 through one of valves 5, and when the piston 2 rises from the lowermost position to an intermediate position (shown by chain line in FIG. 4B) to the uppermost position, the fuel-air mixture is compressed.
With the fuel-air mixture compressed, the spark plug 6 is ignited to cause combustion of the fuel-air mixture, thus moving down the piston 2 to the lowermost position (see FIGS. 4A to 4B to 4C).
When the piston rises again from the lowermost position, burned gas is exhausted from the cylinder 1 through the other of the valves 5 (see FIGS. 4C to 4B (chain line) to 4A). Then, the suction stroke begins again. Due to the four strokes (suction, compression, combustion/expansion and exhaustion), the piston 2 rotates the crankshaft 3 through the connecting rod 4. The rotating crankshaft 3 in turn rotates a rotary shaft 7. If the engine is an automotive engine, the rotary shaft 7 is a drive shaft for driving wheels.
In the combustion engine, while fuel-air mixture is sucked into the cylinder, compressed, burned and exhausted, with the piston 2 reciprocating in the cylinder 1 while kept in contact with the inner wall of the cylinder 1, the inner wall of the cylinder 1 is directly heated by the high-pressure, high-temperature burned gas. In order to prevent damage to the piston 2 and the cylinder 1 due to friction between the outer peripheral surface of the piston 2 and inner wall of the cylinder 1, the piston 2 carries a piston ring 9 which is brought into sliding contact with the inner wall of the cylinder 1. The cylinder 1 is formed with a cooling passage 8 as shown in FIG. 4A to 4C through which water is passed to cool the cylinder 1 and the piston 2 (remove heat from the cylinder and the piston).
By cooling the cylinder 1, thermal energy is lost during the combustion/expansion stroke, so that thermal efficiency deteriorates. For higher thermal efficiency, the portion of the inner wall of the cylinder which is brought into contact with the high-pressure, high-temperature burned gas, which generates power, should be kept at high temperature. On the other hand, the portion of the inner wall of the cylinder that is brought into sliding contact with the piston has to be kept at low temperature for lubrication. However, in the conventional arrangement, since the cylinder is a monolithic member, it is necessary to cool the entire cylinder 1, so that thermal efficiency is low.